Automotive fuel is normally stored at gasoline service stations in large underground tanks. Product lines extend from the tanks to dispensing units which are typically pedestals equipped with dispensing nozzles. A submersible pump delivers the fuel through the product line to the dispenser when a switch at the pedestal is activated to energize the pump. The product line has a check valve so that the product line remains full of fuel at the end of each dispensing cycle. This allows the fuel to be dispensed immediately at the start of the next dispensing cycle. A pressure relief valve relieves the pressure in the product line so that it is maintained at a selected level (typically about 15 psi) at the end of each dispensing cycle. The product line is normally equipped with a leak detector which operates to detect the leakage of fuel from the product line and to close the product line in the event of undue leakage.
While fuel dispensing systems of this type work well for the most part, thermal effects can cause problems. For example, when a service station closes for the night, the fuel in the product line is at approximately the same relatively warm temperature as the fuel in the storage tank. However, the ground temperature around the underground product line can be significantly colder. As a result, the fuel in the product line is cooled and eventually drops to the ground temperature which can involve a temperature drop of 30.degree. F. or more.
As the fuel cools, it thermally contracts and the liquid volume in the product line decreases. The line pressure can in many cases drop below 0 psig, and the vacuum that results causes air infiltration into the product line piping through joints and seals that are not designed to hold a vacuum. The leak detector closes the product line due to the loss of line pressure.
The following morning when the station is opened, a leak check is performed when the pump is started. However, the air that is in the line must be compressed in order to bring the line pressure up to the level needed to open the leak detector valve element. In order to compress the air sufficiently for the leak detector to open, it can require up to ten times the amount of fluid downstream from the leak detector that is required in the absence of air in the line. Consequently, a normal three second leak check can take up to 30 seconds. A customer attempting to dispense fuel during this 30 second period will be unable to do so, as the leak detector interprets the open dispensing nozzle as a leak and remains closed so that the fuel cannot flow to the dispenser.
Although thermal contraction occurs throughout the day in cold climates, it is normally not a problem other than for overnight periods. If the station is busy during the day, the time between successive dispensing cycles is not long enough that thermal contraction can occur to the extent required to create a vacuum. However, premium fuels and other less popular fuels can be dispensed so infrequently that a vacuum can be established and air can enter the product line and create problems even in daytime hours.